Community property applies only to spouses, not to a parent and child, so being in a community property state has no effect on how much of the loss belongs to you. If your share of the ownership of the account is 30%, then you can claim 30% of the capital loss on your own tax return. Your parent claims the other 70%.
If the Form 1099-B for the sale is issued to you, you are a "nominee" for your parent with respect to 70% of any gain or loss. On your tax return you report the full amount of the sale proceeds and cost, with a nominee adjustment for 70% of the gain or loss. The remaining 30% of the loss will be applied against any capital gains on Schedule D in your tax return.
In TurboTax, after you enter the sale with the full amounts shown on the 1099-B, you will first get a screen asking about "less common items" on the 1099-B. If any of the listed items appear on your 1099-B, enter the information. If you don't have any of the listed items on your 1099-B, don't enter anything. A couple of screens later you will come to a screen about "uncommon adjustments." On that screen, check the box that says "Some (or all) of the proceeds from this sale do not belong to me." Then in the box for the adjustment enter 70% of the loss as a positive number. (If you had a gain on the sale you would enter the adjustment as a negative number.) On Form 8949 in your tax return, the adjustment amount will appear in column (g) with code N in column (f).
